micromachines Review Plasmonic Structures, Materials and Lenses for Optical Lithography beyond the Diffraction Limit: A Review Changtao Wang, Wei Zhang, Zeyu Zhao, Yanqin Wang, Ping Gao, Yunfei Luo and Xiangang Luo * State Key Laboratory of Optical Technologies on Nano-fabrication and Micro-engineering, Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China; [email protected] (C.W.); [email protected] (W.Z.); [email protected] (Z.Z.); [email protected] (Y.W.); [email protected] (P.G.); [email protected] (Y.L.) * Correspondence: [email protected]; Tel.: +86-028-8510-0781 Academic Editors: Pei-Cheng Ku and Jaeyoun (Jay) Kim Received: 29 April 2016; Accepted: 27 June 2016; Published: 13 July 2016 Abstract: The rapid development of nanotechnologies and sciences has led to the great demand for novel lithography methods allowing large area, low cost and high resolution nano fabrications. Characterized by unique sub-diffraction optical features like propagation with an ultra-short wavelength and great field enhancement in subwavelength regions, surface plasmon polaritons (SPPs), including surface plasmon waves, bulk plasmon polaritons (BPPs) and localized surface plasmons (LSPs), have become potentially promising candidates for nano lithography. In this paper, investigations into plasmonic lithography in the manner of point-to-point writing, interference and imaging were reviewed in detail. Theoretical simulations and experiments have demonstrated plasmonic lithography resolution far beyond the conventional diffraction limit, even with ultraviolet light sources and single exposure performances. Half-pitch resolution as high as 22 nm (~1/17 light wavelength) was observed in plasmonic lens imaging lithography. Moreover, not only the overview of state-of-the-art results, but also the physics behind them and future research suggestions are discussed as well. Keywords: surface plasmon polaritons; bulk plasmon polaritons; diffraction limit; subwavelength optics; near-field optics; metamaterial; super resolution; nano optical lithography; nanostructure fabrication 1. Introduction Since its invention in about 1960, optical lithography has made a large contribution to the present integrated circuits industry, and has been widely employed in investigations spanning electronics, micro optics, photonics, medical and biology, new materials, etc. The diffraction-limited resolution of optical lithography, usually referred to as about half the wavelength of light, hampers its applications for nano patterns’ fabrication. To solve this problem, light sources with a shorter wavelength are employed in advanced optical lithography, including deep ultraviolet (DUV) of 248 and 193 nm, and even extreme ultraviolet (EUV) of about 13.5 nm [1]. This, unfortunately, requires considerable complex optics, new materials and some other challenging technology problems, and presents a great increase in cost for users and researchers, especially for modest investigations [2]. Nowadays, researchers are reluctantly forced to use high-cost and point-to-point scanning writers, like electron beam lithography (EBL) [3,4] and focused ion beams (FIB) [5,6]. Thus, low-cost, high-efficiency and large-area nanolithography methods are preferred from the viewing point of practical applications. Some novel techniques have been proposed and developed for improved resolution beyond the diffraction limit of conventional optical lithography. For instance, double-exposure techniques Micromachines 2016, 7, 118; doi:10.3390/mi7070118 www.mdpi.com/journal/micromachines Micromachines 2016, 7, 118 2 of 33 were proposed to get doubled dense nano patterns, but with increased expense, complexity and limited patterns [1]. Nano imprinting has been widely explored as a low-cost replica method of nano patterns [7–10]. Near-field lithography deserves special attention due to its inherent relation to the topic of this paper, in which evanescent waves diffracted from subwavelength mask patterns are employed to optically print nano patterns to a photoresist layer in very close proximity and even in a contacting manner [11–13]. This method seems to possess nearly unlimited resolving power in contacting mode but suffers from ultra-short exposure depth and poor fidelity, etc. [13]. Also, some other proposed novel methods show promising but limited nanofabrication ability for periodic patterns and specific materials, like directed self-assembling [14], dip-pen [15] and heat pens [16], etc. Recently, the investigations of and physics behind some abnormal optical phenomena associated with surface plasmon polaritons (SPPs), like extraordinary optical transmission through subwavelength metallic holes array [17], some proposed novel concepts like perfect lens [18] and metamaterial, inspired us and some other research teams to be aimed at plasmonic lithography. The intriguing features of SPPs, including much shorter propagation wavelength than that of light in a dielectric medium with equal frequency and great field enhancement and energy concentration within a subwavelength region, are the main reasons for taking SPPs as a potential way of nanolithography beyond the diffraction limit. As an example, Luo performed subwavelength SPP lithograph experiments since 2003 [19,20], obtaining about 50 nm line-width patterns using SPPs interference and a g-line mercury lamp of 436 nm [21]. Further, partly within the scheme of perfect lens, plasmonic lenses in a variety of forms like super lenses [22], plasmonic reflective lenses [23] and plasmonic cavity lenses [24] were designed and employed to realize imaging lithography with minimum half-pitch resolution down to 22 nm, about 1/17 light wavelength. Some nano devices like metalenses and nano polarizers have been fabricated by plasmonic imaging lens lithography. It is worth notifying that the short exposure depth issue in conventional near-field lithography could be considerably relived in this case, with the help of evanescent waves amplification and resolution-enhanced technologies by engineering the wave front of SPPs in the imaging process [25]. This point is very important from the viewpoint of practical applications, as separating the operational manner could be supported in plasmonic imaging lens lithography. Also, some plasmonic nano focusing structures for converging SPPs energy to a subwavelength point, like localized surface plasmons (LSPs) holes [26], SPP focusing structures with concentric rings [27] and bowties [28], are proposed and successfully demonstrated for point-to-point scanning nano lithography. For plasmonic lithography methods, nano patterns could be fabricated by only employing low-cost, long-wavelength light sources like mercury lamps and He-Cd lasers, etc., fully commercialized and developed materials of resists and no complex optics, just simple illumination of plasmonic lithography structures. Despite some inconvenient operations such as near-field close proximity, it provides a potentially promising nanofabrication lithography method characterized by low-cost, large-area advantages, and SPP imaging and interference lithography is believed to deliver much higher throughput than some conventional nano fabrication tools like EBL and FIB. In this paper, we would like to review the research achievements of plasmonic lithography in the manners of direct writing, interference and imaging. Some representative and important results will be described in details, such as SPPs resonance interference lithography, bulk plasmon polaritons (BPPs) interference lithography, superlens lithography, plasmonic cavity lens lithography, and bowtie lithography, etc. Also discussed are the key aspects in evaluating the performance of plasmonic lithography, such as resolution, fidelity and the aspect ratio of nano patterns. Some new physics and materials accompanying plasmonic devices design and improvements of lithography will be presented as well, including structured light illumination with BPPs, two SPPs absorption and Fano resonance. The remaining problems and outlooks of further investigations of plasmonic lithography will be given in the end. Micromachines 2016, 7, 118 3 of 33 Micromachines2. Characterizations 2016, 7, 118 and Excitations of Surface Plasmons 3 of 32 2.1. SPPs SPPs with Sub-Diffraction Features SPPs exist around the interface between two medi mediums,ums, one of which which has has free free electrons electrons or charges charges and the other does not, as shown in Figure1 1a.a. SPPsSPPs areare formedformed asas electromagneticelectromagnetic fieldfield waveswaves toto excite free charges density oscillation and propagate along interfaces. interfaces. To To view it it in in a a simple simple way, way, the charge densitydensity is is usually usually ignored ignored in the in designthe design and analysis and analysis of plasmonic of plasmonic structures structures by just considering by just consideringmedium with medium negative with permittivity, negative permittivity, which is usually which describedis usually asdescribed the Drude as the model. Drude In model. this way, In thisFigure way,1b Figure indicates 1b thatindicates SPPs that are regarded SPPs are asregarded a special astype a special of surface type of electromagnetic surface electromagnetic wave confined wave confinedat the interface at the ofinterface dielectric of dielectric and metal. and metal. Figure 1. ((aa)) SPPs SPPs (surface (surface plasmon plasmon polaritons) polaritons) on on a a dielectric–m dielectric–metaletal interface interface ( (HH isis in in the the